The Great Lakes may be the world’s largest reservoir of fresh water, but it is also one of the world’s largest test tubes.

That’s the opinion of a Queen’s University biologist who has studied the effects of pollution, humans and invasive species on the Great Lakes.

Dr. Linda Campbell says there is still time to save the Great Lakes — or ruin them.

Campbell, who holds a Canada Research Chair in aquatic ecosystem health, says species have disappeared from the lakes, foreign parasites have gained a foothold and people’s health has been affected by contaminants and chemicals in the water and fish.

It is an ongoing cycle that she says scientists are still trying to understand fully and while she says some measures taken in the past 30 years have improved the quality of the lakes, they remain under threat.

“People hear that the Great Lakes have 20% of the world’s fresh water and they think, ‘We’ve got that much water around, so what’s the problem?’ ” Campbell, who is deaf, said through an interpreter to an interested crowd yesterday at the Marine Museum of the Great Lakes.

The presentation was part of a monthly series of lectures on the Great Lakes and the environment being hosted by the museum.

Campbell said most of the fresh water used by people comes from rainfall or runoff and the usable amount of water is actually a little under 3%, or a little more than 10% of the commonly assumed figure.

“We are living off the interest, not the capital,” she said.

The lakes form a sensitive en-v i ro n m e nt in which small changes can wreak huge havoc, and the lakes always seem to be under some sort of assault, whether it be DDT or phosphates in decades past or the zebra mussels, blood red shrimp and lampreys of today.

Often several threats appear at once and problems can be magnified by problems. Campbell pointed to phosphates, which cause huge growth of algae as one example.

They were banned by government in the 1970s for use in detergents and water treatment and as a result, their levels in fish started to fall but algae blooms fed by phosphates again became a problem in the 1990s, not because of increased human use of the chemical, but because zebra mussels were such efficient feed-e rs that they strained small quantities of it out of the water.

The mussels don’t use the phosphates, so they pooped out the now-concentrated chemical and algae started blooming again near the shore, resulting in the stinking, thick green mats that are familiar to beachgoers in the summer.

DDT was also killing birds but after it was banned, there was an explosion in the almost extinct cormorants.

Poop is also an issue with them — the birds’ feces kill vegetation and denude islands — and the government has had to resort to killing the birds and oiling their eggs to bring the population down.

“When we see something like that happening, we know that something is wrong with the lake,” said Campbell.

A similar threat to DDT exists today. The lakes have high levels of bromines, a ubiquitous fire-retardant chemical used in everything from building supplies to clothing.

Before it was allowed in wide use, scientists used computer models that showed that it would not accumulate in the food chain like earlier chemicals but they were wrong. Levels of the chemicals are rising in aquatic animals with levels, as earlier chemicals, getting higher the further up the food chain you travel.

The various threats, both chemical and biological, have caused the collapse of the Great Lakes fishery, threatened the sport-fishing industry and wiped out entire species such as the blue pike.

At the same time, Campbell said there have been successes. She told the crowd that the price of a healthy ecosystem is constant vigilance and a willingness to address the various and complicated factors at play.

“There are still things we can do,” she said.

“We don’t have to resign ourselves into thinking it is too late.”